The present invention relates to a circuit for processing or reconstructing digital data signals which have been distorted or bandwidth-restricted as a result of being subjected to high- or low-pass filtering.
The necessity of minimizing bandwidth is well known to those of skill in the art of digital data transmission. Numerous schemes have been proposed to achieve minimal bandwidth such as the partial-response filtering process disclosed in U.S. Pat. No. 3,845,412. In such a system, the signals are fed through a low- or high-pass filter. Although this results in a major reduction in bandwidth, it also results in considerable distortion of the digital signal such that a processing circuit is necessary to restore the signal to its original form.
In partial-response filtering, the bandwidth is constrained so that in one clock period the amplitude of modulation can change only by approximately one-half the total amplitude swing. This makes it necessary to provide circuitry to extract the clock pulses from the transmitted signal for switching purposes. Also, a reference voltage must be generated for quantization of the compressed signal.
In addition to the above, the filtering process results in a highly asymmetrical transmitted signal. If low-pass filtering is employed, a consecutive string of logic "1" or "0" bits will produce an increasing voltage swing away from the quantization level. In other words, the first bit of the string will not produce full amplitude swing. The opposite problem is produced in high-pass filtering since the initial amplitude swing for the first bit of the string will be maximum and then decrease for the consecutive bits. The result in either case is a varying differential between the signal level and the quantization level. Where the differential is low, the chance for erroneous quantization and data error is high.
The data error rate is high where the quantization voltage level is fixed. The above-mentioned U.S. Patent utilizes a system in which the quantization or reference level is generated by passing the data signal through a low-pass filter. This has the effect of producing a time average of the signal. However, the time constant of such a circuit must necessarily be relatively long, and for this reason the approach of the above-mentioned prior patent is cannot compensate for a highly asymmetrically distorted signal where the amount of distortion varies on a bit-by-bit basis.